AC-to-DC power converters are used to power a variety of common electronic devices including, e.g., laptop and desktop computers. Such AC-to-DC power converters typically include a diode bridge for rectifying AC voltage from an AC power source, and a DC-to-DC voltage converter for converting the rectified voltage into a DC voltage appropriate for powering a load, e.g., an electronic device. Power factor correction (PFC) is often required for power converters having relatively high power requirements, e.g., drawing greater than 75 Watts from an AC line supply. A common technique for implementing PFC within a power converter is to use a boost converter to convert the rectified voltage into a relatively high intermediate voltage which is then stepped down, e.g., using a buck converter, to a DC voltage as required by the load. A disadvantage of such power converters, with boost PFC or not, is that conduction losses within the diode bridge lead to power inefficiencies and associated heat dissipation requirements. In addition to its added circuit complexity (components), boost PFC incurs additional conduction losses through its electronic devices (switches, diodes) and any passive devices (e.g., energy-storage inductor), thereby leading to further power inefficiencies.
Bridgeless power converters eliminate the diode bridge of conventional power converters by using power switches to effectively rectify the AC power input. The power switches used within such power converters typically can only block current flow in one direction. For example, an N-channel enhancement-mode metal-oxide semiconductor field-effect transistor (MOSFET) conducts current from its drain to its source when a sufficiently high voltage is applied to the MOSFET's gate (control) terminal. If the voltage applied to the gate terminal is not sufficiently high, positive current flow is blocked from the MOSFET's drain to its source. However, an intrinsic body diode within the MOSFET allows current flow from the source to the drain regardless of the voltage applied to the gate terminal, provided the voltage drop from the source to the drain is higher than the body diode's threshold voltage. Hence, the MOSFET is not generally able to block positive current flow from its source to its drain.
The fact that power switches within a bridgeless power converter often cannot block current flow in both directions limits the use of these power switches as control switches for a switching buck and boost converter. The dual use of such power switches for rectification and voltage converter control is not feasible across a broad set of power converter topologies, at least when using minimal circuitry. While more complex circuitry or additional circuit stages might be capable of supporting desired power converter topologies, the additional complexity requires additional and undesirable electrical components, e.g., power switches, diodes, inductors, other magnetics. Furthermore, the additional components often incur additional conduction losses, which negate or at least reduce the efficiency advantage that is sought by eliminating the diode bridge.
AC-to-DC power converter topologies that do not include a diode bridge, use minimal circuitry, can achieve both buck and boost operation and are highly efficient are desired.